[MNG-4632] Class loading is not thread-safe

Fixed problem where ClassWorld was missing a synchronized.

Also code-reviewed synchronization in ClassWorld vs ClassLoader and
discovered no other problems.

git-svn-id: https://svn.apache.org/repos/asf/maven/maven-3/trunk@933714 13f79535-47bb-0310-9956-ffa450edef68

pom.xml

@@ -37,7 +37,7 @@
   <inceptionYear>2001</inceptionYear>
 
   <properties>
-    <classWorldsVersion>2.2.2</classWorldsVersion>
+    <classWorldsVersion>2.2.3</classWorldsVersion>
     <commonsCliVersion>1.2</commonsCliVersion>
     <easyMockVersion>1.2_Java1.3</easyMockVersion>
     <junitVersion>3.8.2</junitVersion>